Extreme ultraviolet light, e.g., electromagnetic radiation having a wavelength of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere herein the term “light” will be used even though it is to be understood that the radiation described using that term may not be in the visible part of the spectrum.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV part of the spectrum. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by using a laser beam to irradiate and so to vaporize a target material having the required line-emitting element to form a plasma in an irradiation region.
The target material may take many forms. It may be solid or a molten. If molten, it may be dispensed in several different ways such as in a continuous stream or as a stream of discrete droplets. As an example, the target material in much of the discussion which follows is molten tin which is dispensed as a stream of discrete droplets. It will be understood by one of ordinary skill in the art, however, that other target materials, phases of target materials, and delivery modes for target materials may be used.
The energetic radiation generated during de-excitation and recombination of ions in the plasma propagates from the plasma omnidirectionally. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to where it is to be used, for example, to a set of scanner optics and ultimately to a wafer in the case where the EUV radiation is to be used for semiconductor photolithography.
The target material is introduced into the irradiation region by a target material dispenser. The target material dispenser is supplied with target material in a liquid or solid form. If supplied with target material in a solid form the target material dispenser melts the target material. The target material dispenser then dispenses the molten target material into the vacuum chamber containing the irradiation region as a series of droplets.
As can be appreciated, one technical requirement for implementation of a target material dispenser is the supply of target material to the target material dispenser. Ideally target material is supplied in a manner that does not require frequent or protracted interruptions in the operation of the overall system for producing EUV radiation, that is, the EUV source. At the same time, because it is desirable to provide for the ability to “steer” the target material dispenser precisely and repeatably (i.e., alter the position of the point at which the target material dispenser releases target material into the vacuum chamber), it is also desirable to provide a target material dispenser that has relatively low mass. There is thus a need to supply the target material dispenser with target material in a manner which does not require undue interruption in the operation of the overall EUV source and which does not add undue mass to the target material dispenser.